The invention refers to a device and a method by which the ion concentration in a solution can be adjusted, regulated and measured amperometrically. The invention relates in particular to a device for adjusting and regulating pH values.
In the field of pH adjustment it is common, especially in laboratories, to adjust pH by titration with solutions (usually acids and bases). Adjustment of the pH value in this way is only possible by increasing volume, which, especially with small quantities of solution, is substantial or difficult to dose. What is more, considerable effort is involved in adjusting unbuffered solutions to an accuracy in the range 1/10 or 1/100 of pH. Diluted acids and bases are used to achieve this accuracy, which contributes further to the drawback of increased volume. Automation is elaborate and complicated because of the means of dosing that are needed. It is also common to titrate ions with the aid of current flow (e.g. Nagy, G. et al., E. Anal. Chim. Acta, 91 [1977] 87). But the device involved presents the disadvantage that electrodes are immersed direct in the solution to be titrated. This can lead to unacceptable electrode reactions with formation of radicals, especially on the anode.
The greatest problems result when adjusting pH values in volumes in the ml region and below, these being indispensable in particular for genetic, diagnostic medical and biochemical methods. Simply filling a solution from one plastic vessel into another produces uncontrolled pH alterations of up to half a unit and more, which can be explained by the surface properties of these vessels (clinging molecules, ion exchanger characteristics, etc). These changes are currently tolerated, or they are countered by elaborate rinsing with large consumption of solution. Added to this is the fact that the pH value often has to be controlled and possibly readjusted in small volumes (especially in diagnostic medical, biotechnical or pure technical routine chores), and here a change in volume and the associated alteration of the chemical solution is entirely unacceptable, or the disadvantages are recognized and tolerated. For this reason, in techniques that must make do with a very limited volume of solution, it is scarcely possible at present to correct pH, which can repeatedly cause inaccuracies in tests and reaction conditions.
Other principles are familiar where pH alterations are produced by electrolytically generated ion currents. Use is made here of the classic principle whereby, in electrolysis of a salt (e.g. NaNO.sub.3), the anode region becomes more acidic because of the accumulation of anions (e.g. NO.sub.3.sup.-) and the cathode region more basic as a result of the accumulation of cations (e.g. Na.sup.+). The cause of this is to be found in the redox reactions on the electrodes.
The transport processes involved in pH shifts are described in more detail in what follows.
In theory, every pH shift--whether intentional or unintentional, whether on electrodes or in a free solution--is based on the fact that, in the solution considered, there is a shift in the difference between the sum of all cations minus the sum of all anions (whereby the H.sup.+ and OH.sup.- ions are to be left out of the calculation). A method that influences the pH value of a solution by electrical means must consequently be able to alter this difference. The necessary prerequisites for this are already to be found in the classic works of Kohlrausch (Ann. d. Phys., 62 [1897] 209), Logsworth (J. Am. Chem. Soc., 67 [1945] 1109) and MacInnes (The Principles of Electrochemistry, Reinhold Publ. Co., New York, 1939) and will be explained taking an NaNO.sub.3 solution as an example:
The Na.sup.+ concentration must be increased and/or the NO.sub.3.sup.- concentration reduced to make the solution more basic. If Regulation is to be by electrical means, this means that the number of Na ions introduced to the solution electrically must be greater than the number of Na ions simultaneously escaping.
In a current-carrying electrolyte there is an inflow of cations (anions) on one side and an outflow of cations (anions) on the opposite side, so the pH value of the solution does not alter. But if an electrolyte phase contains an electrode, there is no longer any ion current flow on the electrode, ie an ion type is only able to either reach or leave the particular region.
The general prerequisite for a change of concentration of a particular ion type in a certain volume is that this ion type exhibit a divergent current flow. According to Kohlrausch, this is the same as saying that the transport number of the particular ion type in the volume considered must not be constant. This is always the case with the electrodes in aqueous systems. The current flow in the electrodes is through electrons, while the current in adjacent electrolytes is produced by migrating ions. For this reason the H current flow exhibits a divergence for example, so there must always be pH shifts in the region of the electrode for current flow in aqueous systems. This mechanism includes the course of redox reactions, and consequently it is ruled out as the basis for a generally applicable ion adjustment system.
If the pH value of a solution containing NaCl is to be adjusted for example (solutions of this kind are widely used in biomedicine), there may easily be formation of hypochlorite or even chlorate ions on the anodes. These ions would have an adverse effect on the solution to be adjusted because they disinfect and bleach (contamination).
Despite this drawback, the redox reactions on electrodes are of considerable interest, so reactions that change the pH value in the region of the electrode through redox processes are also used and investigated (cf Van der Schoot, B., Voorthuyzen, H. and Bergveld, P.: Sensors & Actuators, B1 [1990] 546; Fuhrmann, B., Spohn, U. and Mohr, K. -H.: Biosensors & Bioelectronics 7 [1992] 653, Electrolytic titrating device, DE-PS 15 98 597 [Method and device for determining end point of titration], DE-OS 36 18 520, PCT/GB95/01425 [Improvements in or relating to electrochemical measurements]).
But a controlled pH change through redox reactions is only possible in exceptional cases, like in the system by Shimomura et al. (Shimomura, O.: J. Crystal Growth 144, 253 [1994]; Shimomura, O.: Trans. IECE Japan J 67 C, 673 [1984]; Yokotani, A., Kolde, H., Sasaki, T., Yamanaka, T. and Yamanaka, C.: J. Crystal Growth 67, 627, [1984]). This system relates to amperometric pH control in the growth of KDP monocrystals in saturated solutions in vessels of characteristic dimensions of the order of 20 to 30 cm during the course of several months, and exhibits the following drawbacks. Firstly, pH control to compensate for pH changes in the saturated solution only allows pH change in one direction because of the crystal growth. Secondly, the electrolyte of an anode region bordering on a central adjusting region is mixed with the electrolyte of a cathode region bordering on the adjusting region, with the result that anions from the anode region, especially disturbing hypochlorite ions for example in solutions containing NaCl, can penetrate the adjusting region by way of the cathode region (the same applies to the cations). Such mixing is unacceptable in many applications because of the contaminating effect. Finally, pH change is extremely slow.
A method of pH control in electrolysis processes with pH changes in the solution to be electrolyzed is known from DE-OS 1 571 723. Here there are one or two electrolyte chambers by which H.sup.+ ions and corresponding counterions are injected into the solution to be adjusted to increase the pH value. Alternatively the method can be used to inject OH.sup.- ions.
The method according to DE-OS 1 571 723 presents the following restriction. No pH adjustment or Regulation is possible because pH control is slow and may only be in one direction (either an increase or decrease of the pH value).
In some applications the pH shifts on the electrodes are to be eliminated (e.g. in electrophoresis configurations), while in the following cases they are expressly wished.
In isoelectric focusing a pH gradient is generated over a separation distance by means of current flow and used for protein separation. For this purpose multi-electrode configurations were proposed, the function of which consists in producing a staircase pH gradient in an electrophoretic separaton distance (Hagedorn, R. et al.: DD 273 316; Deml, M., Pospichal, J., Gebauer, P. and Bocek, P.: Czech. Pat., PV 6036-88). This is done by making part of the separation distance acidic or basic at the expense of the other part through the effect of electric fields. A nearly stationary pH gradient can be created, which suits the requirements of electrophoretic separation but, for the following reasons, is unsuitable for pH Regulation. Firstly, the pH shifts are extremely slow because they are initiated solely by diffusion processes and chemical reactions (formation of water). Secondly, formation of the gradient is tied to immiscible media. So this system too is unsuitable for general use in pH Regulation.